Variable interstellar absorption toward the halo star HD 219188: Implications for small-scale interstellar structure

Within the last 10 yr, strong, narrow Na I absorption has appeared at v. si milar to -38 km s(-1) toward the halo star HD 219188; that absorption has c ontinued to strengthen, by a factor of 2-3, over the past 3 yr. The line of sight appears to be moving into/through a relatively cold, quiescent inter mediate-velocity (IV) cloud, because of the 13 mas yr(-1) proper motion of HD 219188; the variations in Na I probe length scales of 2-38 AU yr(-1). UV spectra obtained with the Hubble Space Telescope GHRS in 1994-1995 suggest N(H-tot) similar to 4.8 x 10(17) cm(-2), "halo cloud" depletions, n(H) sim ilar to 25 cm(-3), and n(e) similar to 0.85-6.2 cm(-3) (if T similar to 100 K) for the portion of the IV cloud sampled at that time. The relatively hi gh fractional ionization, n(e)/n(H) greater than or similar to 0.034, impli es that hydrogen must be partially ionized. The N(Na I)/N(H-tot) ratio is v ery high; in this case, the variations in Na I do not imply large local pre ssures or densities.